In general, closure fastening devices for use in connection with plastic bags and the like are known. Furthermore, manufacturing methods for closure fastening devices made of plastic material are generally well-known.
In operation, a closure fastening device for use in connection with a flexible container should be relatively easy to open from the outside, but relatively difficult to open from the inside. Generally, such a container can be used with its interior either under relatively high pressure or under relatively low pressure. The closure fastening device should provide a satisfactory seal for either condition.
Preferably, the closure fastening device should be suitable for economical manufacturing and should be relatively simple in design. In addition, the design should provide for variations in order to meet different needs. For example, it may be desirable to have a closure fastening device which is relatively difficult to open both from the inside and the outside. In general, the closure fastening device, however, should always be relatively easy to close.
In addition, when the closure fastening device is employed with a container, the container may be made from a thermoplastic material and the closure device and sidewalls of the container can be made integrally by extrusion as a unitary piece or can be made as separate components which are subsequently permanently connected together.
However, the thermoplastic resin materials heretofore found practical for the extrusion of interlocking closure devices, and their attachment to films, such as in making containers, have resulted in shrinkage and distortion problems during their use at elevated temperatures. Typical resin materials employed for interlocking closure devices and container films have included polyethylene, polyvinyl chloride copolymers, and synthetic rubbers. However, none of these construction materials have sufficient thermal tolerance for many commercial uses. Further, both occlusion and deocclusion of the interlocking closure device is generally difficult for the user when the device is made from resin materials having high temperature tolerances due to their associated high flexural moduli. Thus, in selecting a resin composition for the profile portions and the flange portions of a closure fastening device which is employed on a bag or container for use in a high temperature environment, such as in a microwave oven or boiling water, the resin composition must meet several criteria. These criteria include high heat resistance, relatively low flexural modulus at low temperatures, acceptable extrusion characteristics, and convertibility of the film into end products such as bags or containers. High heat resistance is necessary because when the bag or container is equipped with the interlocking closure fastening device and is used in a microwave oven where food temperatures can reach about 300.degree. F., or in boiling water where the temperature of the cooking vessel can exceed 212.degree. F., the closure fastening device must retain proper occlusion and deocclusion forces. The resin composition must also be flexible at low temperatures because such bags or containers are often used in a freezer for food storage, and when removed from the freezer, the closure fastening device must be sufficiently flexible so that the bags or containers can be easily opened when such is desired as to remove food therefrom. In addition, the resin composition for the interlocking closure fastening device must have acceptable extrusion characteristics so that it can be easily processed in production and make a reliable, reproducible product.
In the extrusion of such interlocking closure fastening devices, it is desirable to extrude closure elements having base and profile portions onto a common flange portion, separate the closure elements, and then attach the closure elements to the bags or containers. This operation presents a problem in the closure extrusion phase because some of the closure elements are near the outer edges of the flange portion which is an area conducive to "neck-in" of the resin material. "Neck-in" may be described as a decrease in an article's dimensions transverse to the take-off or elongation direction of the article. In such event, the edges of the flange portion exiting from the die will shrink toward the center of the flange portion in a curved path. When the edge of a flange portion "necks-in" toward the center of the flange portion, a closure element positioned near the flange edge will travel with the edge of the flange portion. The result is that the closure element is not extruded in a straight line as desired, but follows the curved path of the edge of the flange portion. This result causes distortion of the closure element due to the two dimensional path followed by the closure element. Therefore, in choosing a resin composition for the flange portion of a closure fastening device as described herein, it is highly desirable to employ a resin composition that suffers a minimum of "neck-in" so that the closure elements extruded near the outer edges of the flange portion travel in as close to a straight line as possible which minimizes closure distortion.
On the other hand, when choosing a resin composition for the closure elements, i.e., the base and profile portions, of the interlocking closure fastening device, the main concern is to employ a composition that will retain the intricate profile shapes of the closure elements during the extrusion process and during the cooling process. This concern is more important than limiting "neck-in" tendencies of the resin composition.